Carbonic Acid Is a Diprotic Acid with K _al =

Carbonic acid is a diprotic acid with K _al = 4.3×10 ^{- 7} and K _a2 = 5.6×10 ^{- 11}. What is K _b for the carbonate ion, CO₃^{2 -} ?

A) 5.6×10³
B) 5.6×10 ^{- 3}
C) 1.8×10 ^{- 4}
D) 4.3×10 ^{- 7}
E) 2.3×10 ^{- 8}